In a semiconductor device such as a MOSFET (metal oxide semiconductor field effect transistor) of the field plate structure, a field plate electrode is provided below a gate electrode. The field plate electrode thus provided relaxes electric field intensity in the drift region of the MOSFET. This improves the drain-source breakdown voltage of the MOSFET. Furthermore, with the improvement of breakdown voltage, the impurity concentration of the drift region can be set higher. This can reduce the on-resistance of the MOSFET. That is, the MOSFET of the field plate structure can improve the tradeoff between the improvement of breakdown voltage and the reduction of on-resistance in the MOSFET.
However, in an actual MOSFET of this type, the electric field concentrates near the p-n junction between the base region and the drift region or the lower end of the field plate electrode. This may hamper the achievement of sufficient breakdown voltage. A better tradeoff is desired between the improvement of breakdown voltage and the reduction of on-resistance in the MOSFET of the field plate structure.